Traditional or conventional audio speakers or loudspeakers are designed to fill a space with sound. This allows for a shared audio experience. Often, however, a person wants to listen to audio in private. This is especially true when the person is using a mobile computing device in a public space. One way to provide private sound to a mobile user (e.g., on a laptop computer or tablet computing device) is by having the user wear headphones. The use of headphones precludes others from listening to the audio. For example, speech that the user or listener is listening to can be kept private.
Parametric speakers (i.e., producing sound from an ultrasonic signal) also provide some level of privacy when used for various audio applications. They have been used to provide a “zone” where sound can be heard by a user that is listening to the audio, without disturbing others. A modulation technique traditionally used with parametric speakers is called square root modulation, and it is essentially equivalent to adding a Direct Current (DC) component to the desired signal (to make it non-negative), and then taking the square root of the results and using standard Amplitude Modulation-Suppressed Carrier (AM-SC) modulation.